Timepieces whose case includes a middle part made of a hard, natural or synthetic mineral such as sapphire, ceramics or a natural or reconstituted jewel, are already known. Such materials do not withstand mechanical stress very well and efforts have therefore been made to reduce machining operations as far as possible or to limit them to machining a housing for a timepiece movement and through passages for the control member stems.
CH Patent No. 517 963 discloses, in accordance with one embodiment, a bezel-middle part type exterior made from a metal carbide based material with a hardness close to that of topaz, while emphasising that it would be very difficult to form threadings in this material for assembling all the other watch components. The same difficulties would be encountered if the material used were topaz or any other natural or synthetic mineral material. In order to overcome this problem of assembling components, it was then proposed to use an inner metallic middle part onto which the bezel-middle part is fitted, while compensating for any variation in dimensions by adhesive bonding and while ensuring water-resistance.
U.S. Pat. No. 4,620,798 also discloses a construction that includes an inner middle part and an outer middle part. The inner metallic middle part both secures a back cover via screws and a bezel via adhesive bonding, which also includes a glued joint for securing the crystal. The back cover and the bezel extend slightly radially beyond the inner middle part and hold the outer middle part, which is made of a natural or synthetic mineral material, in place.
A construction of the preceding type is also disclosed in U.S. Pat. No. 4,853,910, wherein a single part, which forms at the same time the dial, bezel and middle part, made of a hard mineral material, is assembled to an inner metallic middle part by bonding.
As can be seen from the aforecited prior art, adhesive bonding, in one way or another, appears to be an inescapable step for assembling an outer middle part made of a mineral or synthetic material and an inner metallic middle part.
The technique of adhesive bonding has, however, well known drawbacks. The first is the choice of a suitable adhesive for two different materials, such as a metal and a ceramic material. The second is linked to the aging of the adhesive, which can lead to a loss of adhesion, or to cracks that over time can lessen the water-resistance of the case. These drawbacks are not acceptable for medium or top of the range timepieces, which should have a long lifetime.